Oxidation-reduction or simply redox reactions are an important class of chemical reactions encountered in many biological media. Redox reactions occur in living or- ganisms and are indeed essential to life. They are mainly involved in metabolic path- ways to generate energy in the form of free energies. In all living organisms, including microbes, metabolism can be thermodynamically driven by different oxi- dation-reduction systems that can be assessed either by free energy calculation or redox potential (Eh) measurement.
In the digestive tract of animals, microbial activity is partly responsible for physiolo- gical gut conditions such as pH level, Eh and oxygen concentration. The redox conditions that prevail in the gut can have a major impact on the digestion, metabo- lism, and assimilation of ingested nutrients. The oxygen status determines whether anaerobic fermentation or aerobic oxidation of nutrients prevails. Few authors have assessed the redox conditions in all the individual parts of the gastrointestinal tract of animals: Veivers et al. (1980) in termites and Marounek et al. (1987) in goat and sheep. The Eh values in the colon of piglet (Stewart, 1997) and in the caecum of non ruminant herbivores (Kimse et al., 2009) have also been measured. All the ho- meothermic animals previously mentioned have one compartment in the intestinal tract with high fermentation intensity. As a result, bacterial density in rumen, caecum, and colon exceeds 1011/g of digesta and these habitats are anoxic, presenting a high negative Eh value. In general, most of these bacteria are obligate anaerobes and they have the ability to feed themselves on different substrates such as carbo- hydrates, proteins, fibres via processes called fermentation or anaerobic respiration.
The anaerobic fermentation of these substrates goes through the production of me- tabolic intermediates that act as electron acceptors and results in the production of metabolic end-products such as lactate, succinate, short chain volatile fatty acids (acetic, propionic, and butyric acids) and gases, such as H2, CO2 and CH4.
In ruminants, the fermentation compartment called the rumen, contains a plethora of microbes cohabiting and where continuous biochemical processes involving dif- ferent simultaneous chemical reactions occur. Our interest was obviously focused on Eh in such a reducing environment. The first Eh measurements made by Broberg (1958) in sheep ruminal contents revealed values ranging between – 140 and – 260 mV. More recently, Marden et al. (2005; 2008) showed that the rumen contents of dry and lactating dairy cows had a markedly negative Eh that could vary from – 220 to – 115 mV. If the level of dry matter intake (DMI) could partly explain the variation between these values, then the type of diet fed could also influence Eh. A fibre-rich diet is characterized by low Eh values of the ruminal content (Julien et al., 2010a) while a high Eh is observed with a readily fermentable carbohydrates-rich diet. Ac- cording to Julien et al. (2010b), the Eh directly originated from the microbial activity. It reflects an environment with strong reducing potential due to the quasi-absence of oxygen, favorable to strictly anaerobic bacteria.
Recent research studies showed that live yeast used as a dietary feed additive for dairy cows present an intrinsic capacity to reduce the Eh level. This ability enables probiotic live yeast to be another potent modulator of ruminal Eh (Marden et al., 2008a). Live yeast supplementation via the modulation of ruminal Eh can be a good means to stimulate adequate microflora for better digestive efficiency of the diet. However, its effect on ruminal reducing conditions appeared strongly influenced by the level of the original reducing status induced by diet. Future research works may be directed to produce and select appropriate yeast strains with respect to their reducing capacity for use as additives in ruminant diets.
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